Field of the Invention
The present invention relates to a method and an apparatus for manufacturing trichlorosilane by decomposing compounds (hereinbelow, referred to as “polymers”) containing high-boiling chlorosilane, which are generated in a polycrystalline silicon manufacturing process. In particular, the present invention relates to a method and an apparatus for manufacturing trichlorosilane by decomposing polymers that have been separated in a chlorination step or polymers that have been separated from an exhaust gas of a polycrystalline silicon manufacturing process.
Priority is claimed on Japanese Patent Application No. 2008-049229, filed Feb. 29, 2008, the content of which is incorporated herein by reference.
Description of Related Art
The high-purity polycrystalline silicon that can be used as a semiconductor material is mainly manufactured by the Siemens process in which, for example, trichlorosilane (SiHCl3; abbreviated “TCS”) and hydrogen are used as raw materials, a gas mixture thereof is introduced into a reactor and brought into contact with heated silicon rods, and silicon is deposited on the surfaces of the silicon rods due to the hydrogen reduction or thermal decomposition of the trichlorosilane at a high temperature.
The high purity trichlorosilane to be introduced into the reactor, for example, is manufactured by introducing metallurgical grade silicon and hydrogen chloride into a fluidized-bed chlorination furnace to react them, chlorinating the silicon to produce crude TCS (chlorination step), and purifying the crude TCS by distillation into high purity TCS.
In the manufacture of polycrystalline silicon, the reactor exhaust gas includes, in addition to unreacted trichlorosilane, hydrogen, and hydrogen chloride, by-products such as silicon tetrachloride (STC) and chlorosilanes including, for example, tetrachlorodisilane (Si2H2Cl4) and hexachlorodisilane (Si2Cl6). (Patent Document 1: PCT International Publication WO 02/012122) The chlorosilanes have boiling point higher than that of silicon tetrachloride, and they are referred to herein as “high boiling chlorosilanes”. Also, the gas produced in the chlorination reactor includes, in addition to trichlorosilane and unreacted hydrogen chloride, by-products such as silicon tetrachloride and high-boiling chlorosilanes.
Conventionally, when the gas that has been produced in the chlorination reactor is purified by distillation, separated polymers undergo a hydrolytic process and are then discarded. The exhaust gas of the reactor is introduced into a recovery distillation column, and after the polymers have been separated, the exhaust gas is returned to the distillation step of the chlorination step and recycled. However, the separated polymers undergo a hydrolytic process and are then discarded. Thus, there is problem in that the hydrolytic and the waste disposal processes are costly.
A method is known in which the polymers generated in the manufacture of polycrystalline silicon are returned to the fluidized-bed chlorination furnace and then decomposed and used in the manufacture of trichlorosilane (Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H01-188414). However, in this method, because the silicon powder and polymers supplied to the chlorination reactor are mixed, there is a problem that the fluidity of the silicon powder is reduced and the conversion rate of the silicon powder to chlorosilanes is lowered.